1. Field of the Invention
The present invention relates to a base station for transmitting a pilot signal at a predetermined transmission power, a mobile station for transmitting a preamble prior to a start of transmission/reception of data, a communication system for implementing radio communication based on code division multiplexing between the base station and the mobile station, a transmission control method, and a mobile station control program.
A “signal interference ratio” in the present specification means a power ratio of a signal to an interference signal with the signal. The interference signal herein can be the sum of the so-called interference signal (Interference) and noise signal (Noise) and, in this case, the signal interference ratio is called SINR (Signal Interference & Noise Ratio). The interference signal can also be defined by adopting the so-called interference signal (Interference) only and, in this case, the signal interference ratio is called SIR (Signal Interference Ratio). Embodiments of the invention described later will explain examples adopting SINR.
2. Related Background Art
The preamble power ramping is applied to random access of W-CDMA (cf. “W-CDMA Mobile Communications System” pp 130–134 (editorship: Keiji Tachikawa, publisher: Maruzen Co., Ltd.)). A preamble is a short signal transmitted prior to transmission of data packet, and a spreading code to be used is selected from sixteen types of limited spreading codes. Therefore, a base station can readily detect a preamble by means of a simple matched filter. Furthermore, the application of power ramping using this preamble can reduce the adverse effect of interference on other users due to control error of open-loop transmit power control. Specifically, a mobile station repeatedly transmits the preamble plural times until it receives an AI (Acquisition Indicator), which indicates detection of the preamble, from a base station. The mobile station gradually increases its transmission power every transmission of preamble. The mobile station terminates the transmission of preamble at a time of reception of an AI and starts transmitting a data packet at a power corresponding to a preamble transmission power at that time. The mobile station selects a spreading code to be used, at random every transmission of a preamble, and, when receiving an AI, it starts transmitting a data packet with a spreading code uniquely corresponding to a spreading code used for the last preamble, from a determined time (e.g., 4 ms) after the transmission of the last preamble.